My initial impression is, "Wow! Very nice." Overall lens construction is excellent. Manual focus and zoom rings are smooth and a bit stiff (as they should be, esp. for new); no hysteresis or 'deadzone' slip. No weather sealing rubber gasket at lens mount. Lens body has very close-fitting components. I'll use a TT raincoat in the rain. Same as for my 'weather sealed' L lenses. Plastic hood is a bit 'plasticy', but then it is plastic; it's light and should be OK. I added a 6" Hejnar general-purpose A-S rail with "backstop" to the tripod mount foot, and it's pretty solid. The back edge of the tripod mount foot is flat and perpendicular to the sides of the foot. As discussed elsewhere, a two-screw tripod mount foot would be even better. For anybody who cares, it has a 9-bladed "outie" diaphragm. The earlier non-OS was an "innie" niner. As posted back in Feb 2012*, it works with Canon EF 1.4x III and 2x III Extenders.

It weighs virtually the same as my EF 200/1.8L, but it feels a lot less awkward. The EX has much better balance on a 1D-series body - not nose-heavy like the 200/1.8L. I could shoot handheld all day with this lens on a 1D body, but I'll probably use it on a monopod most of the time, because I'll often shoot with at least one other body (70-200/2.8) on a shoulder strap. Speaking of shoulder strap, the EX lens does not have strap lugs. It's a bit too heavy to hang off the camera strap lugs. I'll probably put a simple nylon webbing sling/strap (aka hero loop) around the 'neck' of the tripod mount foot, and use a carabiner to clip it onto a cross-body sling, for both handheld and monopod use (keeps the monopod from falling over while you shoot with the other lens).

Here's a few first impression example photos. I'll post four, two-panel images. All at 300mm focal length. In each case, the top panel shows a 100% crop view of the area highlighted in red in the bottom panel, which is the PS CS6 Navigator window. These screen captures of RAW file images, with auto-contrast as the only PP (no sharpening). A little more PP for contrast and sharpening would make these images "pop" a lot more.

The first image is on a 1DX with no Extender, at very close to the 2.5m MFD for 300mm; handheld at f/2.8, 1/250 sec, ISO 800, indoors in moderate ambient light.

The following three images are on a 1DIV with 1.4x III Extender, on a monopod, at f/8 (lens set at f/5.6), ISO 800, and 1/2000 sec for the second and third image (from the same RAW file), and 1/4000 sec for the fourth image.

The lens appears to have very well-controlled CA (as mentioned in many reviews). Sharpness with a 1.4x III Extender varies from stinking sharp in the center to pretty darn sharp in the corners. Of course, this is just day one. Now I have to go and do something that I'll actually get paid for.

The air is pretty humid here today (not uncommon, around here), and so there are definite atmospheric effects apparent in the example photos. Despite this, I believe that you can see the high performance of this lens. Maybe I'll post some 1951 USAF lens test chart results later. You think you've heard complaining...

Glad you are enjoying yours, Jim.
I am quite happy with mine, will probably add some of my tests, if you don't mind.
While I am happy with the AF performance on still objects (it's fast and accurate), on moving subjects I am not so sure.
I need further testing, but I thinks it's a bit lacking for a sports lens.
The HSM itself is quite fast, not latest USM fast but still very fast.
I believe the problem lies with the reversed engineered firmware.
It takes longer for calculations to be made.
Case in point: I also have a sigma 85mm 1.4 that is blazingly fast for a lens of this focal length and aperture. It's incredibly faster than the canon counterpart, and while it's true that the canon has a wider aperture (more glass to move), the difference is not all that much, since side to side, the canon is not much bigger. But the AF is. Sigma's insanely faster. But many times the glass moves (very fast), then it hesitates a bit as in trying to guess what to do, and then moves again.
The 120-300 also seems to somewhat hesitate, to a lesser degree but it still does it a little, as in trying to guess what to do (where to focus, i.e. where the subject is moving).
This is firmware related, I believe.
Mechanically speaking, the HSM as the potential to be a great performer.
This is on moving subjects, of course. On stationary, no problems whatsoever.

I also noticed that the 1.4 tc (I bought a sigma) adds a lot of vignetting at f4. It goes away at f5.6. It's a very noticeable difference between these two apertures. I am not a guy to notice vignetting, but in this case, it's quite noticeable.

MSC wrote:
Are you happy with the sharpness compared to Canon lenses then? Sorry the basic question but that is a tempting lens...I'm so picky and used to Canon lenses.

I'm picky too. It's too early for me to answer your question. I hope that the 120-300 OS will replace the 300/2.8L IS that I sold earlier to fund my X-Pro 1 system (no regrets, except I regret selling the lens...). I don't think it will be as sharp as the 300/2.8L IS, and won;t be able to compare them ( ), but I still have plenty of ubersharp glass, and so I'll get a good understanding, over time.

MSC wrote:
It is handholdable? For perspective, I think the 70-200 2.8 is and the 300 2.8 not really. What do you think?

Yes. Easily handholdable. OTOH, I'm probably more like RobDickinson, as I can handhold my 500/4L IS for a half-day's shooting at the racetrack; although it spends most of it's time nose down on the ground, or cradled in my crossed arms...

P.S. too bad about the canoe-related incident. As you said, it's the people that count!

MSC wrote:
It is handholdable? For perspective, I think the 70-200 2.8 is and the 300 2.8 not really. What do you think?

Being handholdable depends greatly on the person and the duration and type of shooting.
Some people are stronger than others.
Some people are strong but don't have much stamina (muscles get tired fast).
Some people might be strong but have injuries.
Age can also play a factor.

And, of course, handholdable... for how long? In which conditions?
For a few minutes, hours, half a day? Can you rest once in a while for a few seconds or do you have to keep it at eye level all the time?
It all adds up and makes a difference.

As an example, I see people complaining that the 70-200 2.8IS II is heavy.
It is a toy for me.

The 120-300 is not a toy.
It's big and it's heavy.
Is it handholdable?
For me, it is. But I lift wheigts and I'm fairly strong.
But even then I will feel the weight after a while and it does put some strain on my left wrist. Now I use a wrist band when I go out with it.
But that is me.
My wrists have seen better days. See what I mean?

fraga wrote:
Being handholdable depends greatly on the person and the duration and type of shooting.
Some people are stronger than others.
Some people are strong but don't have much stamina (muscles get tired fast).
Some people might be strong but have injuries.
Age can also play a factor.

And, of course, handholdable... for how long? In which conditions?
For a few minutes, hours, half a day? Can you rest once in a while for a few seconds or do you have to keep it at eye level all the time?
It all adds up and makes a difference.

As an example, I see people complaining that the 70-200 2.8IS II is heavy.
It is a toy for me.

The 120-300 is not a toy.
It's big and it's heavy.
Is it handholdable?
For me, it is. But I lift wheigts and I'm fairly strong.
But even then I will feel the weight after a while and it does put some strain on my left wrist. Now I use a wrist band when I go out with it.
But that is me.
My wrists have seen better days. See what I mean?

For example I can easily hand hold the 500 mk I at my eye for say 5-10 minutes, but I then find I need to rest it at my side. Doing this I can hand hold it for hours. But I could not keep it at eye level for too long. Even the comparatively lightweight 300 f/2.8 gets tiring at eye level after about 15 minutes, but I can walk around with it all day if I put it in the resting position between shots. But like you I go the gym and lift decent weights.

Pixel Perfect wrote:
For example I can easily hand hold the 500 mk I at my eye for say 5-10 minutes, but I then find I need to rest it at my side. Doing this I can hand hold it for hours. But I could not keep it at eye level for too long.

Your experience with the 500 mirrors mine exactly.
I have shot with one for hours handholding it.
But after some minutes at eye level I have to lower it, at least for a few seconds or eventualy even a minute or two as time goes by, to give my arms and shoulder a little break.
I can go for hours like this.
But I'm sure many can't, for several reasons.
Just like some can hold it at eye level much longer than me.

Saying a lens is or isn't handholdable is a very personal opinion and depends greatly on the intended use.

Jeff_Stapleton wrote:
Do you think its a viable replacement for something like a 300/2.8 prime?

For me, it's a temp. Maybe it will be a replacement for the EF 300/2.8 L IS, but I'm certainly not prepared to say that, now. OTOH, I have high expectations for this lens; otherwise, it would not be an acceptable 'temp'. TWT.

IOW, I can't currently afford another 300/2.8L IS to replace the one I sold earlier this year, but I really need a 300/2.8. I expect it's a good facsimile for most situations, when you're not using an extender. I hope it's an acceptable replacement for the 300/2.8L IS with a 1.4x III Extender. So far, so good, but it's too soon to say. OTOH, based on all of the info I can find from online reviews and etc. (links in a previous thread), I expect it's usefully better than than all of the alteratives - including Sigma, Tamron, and Tokina AF 300/2.8 prime lenses, despite its relatively reasonable price. As I said, so far, so good.

Jeff_Stapleton wrote:
Do you think its a viable replacement for something like a 300/2.8 prime?

Greatly depends on your expectations.
It is not as sharp as a 300 2.8IS.
Then again, I don't think anyone expects it to be, for a myriad of reasons.
If it's sharp enough for your purposes, then it is a viable solution, as it can zoom and is much cheaper.

P.S. if the opportunity ever arises, you should tell the cox'n on that high speed craft (HSC) to tell his boss that he should look into "shock mitigation seats". Could save a bundle, in the long term...

fraga wrote:
Hope all of these help people who are contemplating it decide if it's worth it or not.

It's always a personal decision. The best way to find out, is to try it. Same could be said for any lens, regardless of price. Of course expectations increase with price, except for those unhappy people who require prefection, regardless of price... They're still looking, and always will be (as am I).